The present invention relates to a ceramic honeycomb structure. More particularly, the present invention relates to a ceramic honeycomb structure capable of balancing the disadvantages incurred by the increased pressure loss and the decreased thermal shock resistance against the advantages brought about by the increased isostatic strength and the cell wall shape and honeycomb external shape of higher accuracy, and which is suitably used, for example, as a carrier for catalyst for automobile exhaust gas purification. The ceramic honeycomb structure of the present invention is also used suitably as a filter for diesel particulates or the like, as a chemical reactor (e.g. a catalyst carrier for fuel cell reformer), or as a heat exchanger.
As catalysts for purification of automobile exhaust gas, there are used so-called honeycomb catalysts wherein a catalyst component is loaded on the surfaces of the cell walls of a ceramic honeycomb carrier (honeycomb structure). In these catalysts, since the axial direction strength of the honeycomb carrier is higher than its strength in the sectional (diameter) direction, the honeycomb carrier was held in the axial direction. In this holding manner, in order to prevent the breakage of the honeycomb carrier occurring at around the periphery of its ends in axial direction holding, the thickness of the cell walls (ribs) near the circumference of the honeycomb carrier was made larger than the thickness of the cell walls in the inner portion of the honeycomb carrier to increase the anti-pressure strength of the honeycomb carrier in the axial direction.
Recently, however, the higher output adopted in engines has required a lower pressure loss of honeycomb catalyst and the stricter regulation employed for exhaust gas has needed the effective utilization of whole catalyst carrier; therefore, it has been started to hold the honeycomb carrier mainly at its outer surface, in place of holding in the axial direction. Another reason for this is that the stricter regulation for exhaust gas has invited a larger catalyst volume and an increased catalyst mass and, as a result, the holding in the axial direction has become unable to give a sufficient holding area and promise sufficient holding relative to engine vibration.
Meanwhile, in order to enhance the purification ability of catalyst, it has been started to make thinner the thickness of the cell walls of a honeycomb carrier and decrease the weight of the honeycomb carrier and thereby reduce the heat capacity of a catalyst and enhance its purification ability (warm-up property).
The above use of cell walls of thinner thickness tends to result in a lower fracture strength against the external pressure which the honeycomb carrier receives at the outer surface.
In order to meet the recent even stricter regulation for exhaust gas, it has been aimed to improve the conditions of engine combustion and the purification ability of catalyst. As a result, the temperature of exhaust gas has become higher year by year and the thermal shock resistance required for a honeycomb carrier has become stricter.
Thus, due to the thinning of the cell walls, the employment of holding honeycomb carrier at the outer surface, the increase in temperature of exhaust gas and the like, the setting of cell wall thickness and honeycomb outer wall thickness, the increase in the isostatic strength of honeycomb structure, and the higher accuracies of honeycomb external shape and cell wall shape have become important tasks to be achieved.
In connection with the above, there was proposed, in JP-A-54-110189, a honeycomb carrier structure whose cell walls are made thinner at a given ratio from the outermost peripheral cell wall towards the center of the cross-section. In this structure, since use of a thin wall in the entire honeycomb carrier is impossible, the total mass of the honeycomb carrier is inevitably large, posing a problem in the warm-up property of the honeycomb carrier. This structure is undesirable also in pressure loss.
There was also proposed, in JP-A-54-150406 and JP-A-55-147154, a structure wherein the walls of the cells near the circumference of the structure are made thicker than those of the inner cells. However, no mention is made on the thickness of the outer wall or on the specific relation between different cell wall thickness therein.
In these honeycomb structures of the prior art, since the thickness of inner cell walls is as large as 0.15 mm or more and the holding is made in the axial direction, the thickness of the honeycomb outer wall was not a problem. One may merely point out that too large an outer wall thickness gives a low thermal shock resistance, if forced to do so.
Further in WO 98/05602 was proposed a ceramic honeycomb structure wherein the average cell wall thickness T is 0.05 to 0.13 mm, the average outer wall thickness is larger than T, W greater than T (W is an average width of contact between cell wall and outer wall), and 0.7xe2x89xa7xe2x88x92(T/4)+0.18.
This ceramic honeycomb structure exhibits some effect in prevention of peripheral chipping during handling; however, it was not fully satisfactory in increased pressure loss, reduced thermal shock resistance, increase in isostatic strength, and the improvements in the accuracies of cell wall shape and honeycomb structure external shape.
No in-depth investigation has hitherto been made particularly on the improvements in the accuracies of cell wall shape and honeycomb structure external shape. That is, a ceramic honeycomb structure is generally molded by extruding, for example, a cordierite raw material for ceramic through a die having lattice-shaped slits; then dried; and fired to become a product. When a smaller cell wall thickness is employed, the cell walls tend to deform during molding, owing to the cause mentioned later and resultantly the fired material obtained showed no satisfactory isostatic strength while this did not happen when the cell wall thickness was as large as 0.15 mm or more. Nevertheless, no sufficient investigation has been made. The deformed cell walls are destroyed at the deformed sites by a small force. That is, when cell walls do not deform and are molded at a high accuracy, they theoretically become sites of compression stress when a pressure is applied to the outer surface of honeycomb structure, and the destruction of honeycomb structure takes place owing to the buckling of cell wall or outer wall. Meanwhile, when cell walls have deformed, a bending stress (a stress in tensile direction) is generated at the deformed sites, resulting in easy destruction. In general, materials are less resistant to tensile strength than to compression stress and, in ceramic materials, in particular, the ratio (about 1/10) of tensile strength to compression strength is very small as compared with that (about 1/3) of metal materials. Therefore, when there is deformation of cell walls, destruction takes place at a strength considerably lower than a strength at which destruction takes place ordinarily.
The present invention has bee made in view of the above problems, and aims at providing a ceramic honeycomb structure capable of balancing the disadvantages incurred by the increased pressure loss and the decreased thermal shock resistance against the advantages brought about by the increased isostatic strength and the cell wall shape and honeycomb external shape of higher accuracy, and which is suitable particularly as, for example, a carrier for catalyst for automobile exhaust gas purification.
In order to achieve the above object, the present inventor made a study including various tests mentioned later, with considering the thinner cell walls recently employed in honeycomb carriers. As a result, the following was found out. That is, the adoption of a thick wall only in the cells near the circumference of honeycomb structure as seen in the prior art is insufficient and attention must be paid also to the extrudability of honeycomb structure; therefore, the designing of a honeycomb structure need be made while paying attention not only to the relation between the wall thickness of each outermost peripheral cell and the wall thickness of inner cells (basic cells), i.e. the basic cell wall thickness, but also, while considering the basic cell wall thickness and the thickness of honeycomb outer wall, to the relation between the basic cell wall thickness and the wall thickness of the cells existing between the outermost peripheral cell taken as starting cell and any cell taken as end cell of a certain number of cells extending inwardly from the starting cell and located near the circumference of honeycomb structure; by making the designing of a honeycomb structure as above, the above-mentioned aim of the present invention can be achieved. The present invention has been completed based on the above finding.
It has heretofore been believed that a honeycomb structure having a high strength against a pressure applied to the outer surface can be obtained by allowing the honeycomb structure to have an increased outer wall thickness. There were produced cordierite-made thin wall honeycomb structure samples having an outer diameter of 90 mm, a length of 110 mm, a square cell shape, a cell wall thickness of 0.11 mm and a cell density of 9.3xc3x97105 cells/m2 (wall-to-wall distance: 1.04 mm), with varying the outer wall thickness between 0.1 and 0.9 mm; and they were measured for isostatic strength and the results are shown in FIG. 5. As shown in FIG. 5, the isostatic strength showed no increase and reversely decreased, even if the outer wall thickness was made thicker than 0.4 mm.
The reason for the fact that mere increase in outer wall thickness gives no increase in isostatic strength, is considered to be that as the outer wall thickness is increased, the amount of shape deformation of the wall (rib) of the cells near the circumference of honeycomb structure increases and moreover the number of the deformed walls increases. This is considered to be because as the outer wall thickness is increased, the amount of the raw material passing, during extrusion molding, through the die slit for outer wall formation increases and, as a result, the ribs of the cells near the circumference of honeycomb structure are dragged toward the outer wall and the raw material flow in outer wall and the raw material flow in ribs become unbalanced. The main reasons therefor are considered to be that the change of ribs to smaller thickness incurs easy buckling deformation and that, when the honeycomb structure after extrusion molding is fixed by a jig at the outer surface, the honeycomb structure per se deforms owing to the own weight and, as a result, deformation of the outer wall and the inside ribs, particularly the ribs near the circumference of honeycomb structure takes place. This tendency is considered to be higher as the ribs are thinner and the structure is bigger.
According to strength of materials, buckling strength is basically proportional to the square of cell wall (rib) thickness, as shown by the following formula. It is appreciated from the formula that the thickness of cell wall has a great influence on the strength of honeycomb carrier.
Buckling strength=(kxcfx802E)xc3x97(t/L)2 
wherein k is a coefficient, E is a Young s modulus, L is a cell wall length, and t is a cell wall thickness.
Also, there was conducted a test for thermal shock resistance in supercooling, wherein a honeycomb carrier having a cell wall thickness of 0.11 mm was heated in an electric furnace for a given length of time to make the temperature uniform and then taken out from the furnace. The results are shown in FIG. 6. As the outer wall thickness of the honeycomb carrier was increased, the thermal shock resistance decreased and the decrease was larger when the outer wall thickness was 0.7 mm or more. This is considered to be that as the outer wall is thicker, the outer wall per se has a larger heat capacity and the temperature difference between inside and outside the outer wall is bigger.
For lowering the heat capacity of outer wall, there is an idea of forming notches in the outer wall, as seen in the above-mentioned JP-A-54-150406. This idea has a meaning if the outer wall is sufficiently thick; however, when the cell wall thickness is very thin (1.12 mm or less), the outer wall cannot be freely made thicker as mentioned previously and resultantly the effect of notches is small. Conversely, there is a risk of decreasing the rigidity of the outer wall.
Also, strength was measured in honeycomb carriers wherein the wall thickness in the cells near the circumference of the carriers were made larger than the inner cell (basic cell) wall thickness as in the prior art. As a result, there was certainly an increase in strength; however, as the wall thickness in the cells near the circumference of the carriers was too large, the strength tended to decrease. Inspection of a carrier having a considerably large wall thickness in the outermost peripheral cell indicated that the cell wall of the outermost peripheral cell had deformation. The reason is considered to be the same as the reason for the above-mentioned fact that mere increase in outer wall thickness gives no increase in isostatic strength. As in the case of mere increase in outer wall thickness, mere increase in the wall thickness in the cells near the circumference of carrier does not necessarily contribute to the increase in isostatic strength.
FIG. 7 shows the results obtained when each cell wall thickness of the cells existing between each outermost peripheral cell taken as a starting cell and any cell of the 2nd to 20th cells extending inwardly from the starting cell was increased from the basic cell wall thickness (75 xcexcm) to 100 xcexcm, 150 xcexcm or 200 xcexcm and each of the resulting honeycomb structures was measured for isostatic strength (%). As can be taken from FIG. 7, the degree of increase in isostatic strength was low when the wall thickness of the first to 4th cells was made larger; a striking increase in isostatic strength was seen when the wall thickness of each cell from any of the 5th to 15th cells was made larger; and there was observed the settlement in the degree of increase in isostatic strength even if the wall thickness of the 5th to 20th cells was made larger. No increase in isostatic strength was seen when the wall thickness of the 1st to 2nd cells was made larger; however, a sign of increase was seen when the wall thickness of the 1st to 3rd or 4th cells was made larger; and a clear increase was seen when the wall thickness of 5th or later cells was made larger.
Strength measurement was also made by changing the dimension of external shape of honeycomb structure. In a honeycomb structure having a circular section of 144 mm or more in diameter or having an elliptical section having the same sectional area as the circular section, an increase in strength was seen when the wall thickness of each cell of the 10th to 30th cells was made larger; and there was observed in a similar tendency to that mentioned above in the degree of increase in strength when the wall thickness of the 10th to 40th cells was made larger. (This was the same tendency as seen above.)
FIG. 8 shows the results obtained when each cell wall thickness (Tr1xcx9cTr13) of the outermost peripheral cell, taken as a starting cell to the 13th cell was made larger one by one than the basic cell wall thickness (Tc) so as to give a ratio of 1.00 to 3.00 in terms of [(Tr1xcx9cTr13)/(Tc)] and measurement of isostatic strength (%) was made. As seen from FIG. 8, a sharp increase in isostatic strength was seen from when the ratio of said each cell wall thickness to basic cell wall thickness, i.e. [(Tr1xcx9cTr13)/(Tc)] was 1.10; and the degree of the increase settled when the [(Tr1xcx9cTr13)/(Tc)] was 2.5.
FIG. 9 is a partly enlarged view of FIG. 8. As seen from FIG. 9, since the sharp increase in isostatic strength lasts to [(Tr1xcx9cTr13)/(Tc)] =1.20, it is preferred to adopt a [(Tr1xcx9cTr13)/(Tc)] of 1.20 or more.
FIG. 10 shows the results obtained when each cell wall thickness (Tr1xe2x80x94Tr13) of the 1st cell (taken as starting cell) to the 13th cell was made larger one by one than the basic cell wall thickness (Tc) so as to give a ratio of 1.00 to 3.00 in terms of [(Tr1xcx9cTr13)/(Tc)] and measurement of pressure loss (%) was made. As seen from FIG. 10, a sharp increase was seen from when the [(Tr1xcx9cTr13)/(Tc)] was 3.00; therefore, it is preferred to adopt ordinarily a [(Tr1xcx9cTr13)/(Tc)] of 3.00 or less and, from practical viewpoints, a [(Tr1xcx9cTr13)/(Tc)] of 2.50 or less, preferably 1.60 or less.
FIG. 11 shows the results obtained when each cell wall thickness (Tr1xcx9cTr15) of the outermost peripheral cell taken as a starting cell to the 15th cell was made thicker than the basic cell wall thickness (Tc) so as to give a ratio of 2.0 in terms of [(Tr1xcx9cTr15)/(Tc)] and measurement of pressure loss (%) was made (case 1); when, in addition to the conditions of the case 1, each cell wall of the 16th to 20th cells was allowed to have a sectional shape of an inverse trapezoid (the minor base was present inwardly), the thickness of said each cell wall was made thinner as said each cell wall was located more inwardly, and the thinnest wall thickness was made identical to the basic cell wall thickness (Tc) (case 2); when, in addition to the conditions of the case 1, each cell wall of the 16th to 20th cells was allowed to have a sectional shape of a spool, the thickness of said each cell wall was made thinner as said each cell wall was located more inwardly, and the thinnest wall thickness was made identical to the basic cell wall thickness (Tc) (case 3); when the ratio of the outermost peripheral cell wall thickness (Tr1) to the basic cell wall thickness (Tc) was set at 2.0, each cell wall of the 2nd and later cells was allowed to have a sectional shape of an inverse trapezoid (the minor base was present inwardly), the thickness of said each cell wall was made thinner as said each cell wall was located more inwardly, the smallest wall thickness was made identical to the basic cell wall thickness, and measurement of pressure loss (%) was made (case 4); and when the ratio of the outermost peripheral cell wall thickness (Tr1) to the basic cell wall thickness (Tc) was set at 2.0, each cell wall of the 2nd and later cells was allowed to have a sectional shape of a spool, the thickness of said each cell wall was made smaller as said each cell wall was located more inwardly, the thinnest wall thickness was made identical to the basic cell wall thickness, and measurement of pressure loss (%) was made (case 5). As seen from FIG. 11, pressure loss is large in the cases 1 to 3; therefore, when increase in pressure loss is a disadvantage, it is preferred to make gradually smaller the cell wall thickness from the outermost peripheral cell toward inner cells as in the cases 4 and 5.
FIG. 12 shows the results obtained when, in the cases of FIG. 11, thermal shock resistance (%) was measured in place of pressure loss. As seen from FIG. 12, when the cell wall thickness was made gradually smaller from the predetermined cell to a particular inner cell as in the cases 2 to 5, an increase in thermal shock resistance can be obtained as compared with the case 1.
FIG. 13 shows the results obtained when pressure loss (%) was measured by setting, at 2.0, the ratio of each cell wall thickness (Tr1xcx9cTr30) of the cells existing between the outermost peripheral cell taken as a starting cell and any cell extending therefrom to the 30th cell, to the basic cell wall thickness (Tc), i.e. [(Tr1xcx9cTr30)/(Tc)]. As seen from FIG. 13, pressure loss increases from when the number of cells of thickened wall exceeds 20.
FIG. 14 shows the results obtained when external shape accuracy (mm) was measured by setting one by one, at 1.6, the ratio of each cell wall thickness (Tr1xcx9cTr20) of the cells existing between the outermost peripheral cell taken as a starting cell and any cell extending therefrom to the 20th cell, to the basic cell wall thickness (Tc), i.e. [(Tr1xcx9cTr20)/(Tc)]. As seen from FIG. 14, external shape accuracy (dimensional accuracy) increases from when the number of cells of thickened wall exceeds 5 and, when the cell walls of up to the 15th cells are made thicker, the dimensional accuracy is half of when the cell wall thickness is constant and the same as the basic cell wall thickness. The reason is considered to be that the thicker wall thickness adopted in the cells near the circumference of honeycomb structure increased the rigidity of the structure and the deformation occurring from structure molding to its firing was suppressed. It is considered that this also contributes to the improvement in the uniform molding.
Based on the results of the above study, there is provided the following ceramic honeycomb structure according to the present invention.
1A ceramic honeycomb structure (1) constituted by cell walls (ribs) (2) forming a composite structure from a plurality of cells (3) being adjacent each other and a honeycomb outer wall (4) surrounding and holding the outermost peripheral cells located at the circumference of the composite structure;
characterized in that a basic thickness of cell walls (2) (the basic cell wall thickness) (Tc) is Tcxe2x89xa60.12 mm, an outer wall thickness (Ts) of the honeycomb structure is Tsxe2x89xa70.05 mm, and an open frontal area (P) is Pxe2x89xa780%, and there is a relation shown by a formula:
1.10xe2x89xa6(Tr1xcx9cTr3-20)/Tcxe2x89xa63.00 
between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr3-20) of cells existing between an outermost peripheral cell and any cell within a first end cell from a third cell to a twentieth cell extending inwardly, taking the outermost peripheral cell as a first starting cell.
[2] A ceramic honeycomb structure according to the above [1] wherein there is a relation shown by a formula:
1.10xe2x89xa6(Tr1xcx9cTr3-15)/Tcxe2x89xa63.00 
between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr3-15) of cells existing between an outermost peripheral cell and any cell within a first end cell from a third cell to a fifteenth cell extending inwardly, taking the outermost peripheral cell as a first starting cell.
[3] A ceramic honeycomb structure according to the above [1] or [2], wherein any cell within a second end cell from a third cell to a fifth cell extending inwardly, taking a cell adjacent to the first end cell but located inward therefrom as a second starting cell, has such a cell wall thickness that a section of said each cell wall has a rectangular shape whose minor side of rectangle is a cell wall thickness thereof when the honeycomb structure is cut by a plane perpendicular to the direction of the cells (passages),and
a cell wall thickness having a shortest minor side is identical to the basic cell wall thickness (Tc), by shortening a minor side thereof one by one as a cell is located more inwardly.
[4] A ceramic honeycomb structure according to the above [1] or [2], wherein any cell within a second end cell from a third cell to a fifth cell extending inwardly, taking a cell adjacent to the first end cell but located inward therefrom as a second starting cell, has such a cell wall thickness that a section of said each cell wall has such an inverse trapezoidal shape as a minor base of inverse trapezoid is a thickness of said each cell wall when the honeycomb structure is cut by a plane perpendicular to the direction of the cells (passages), and
a thickness of a cell wall having a shortest minor base is identical to the basic cell wall thickness (Tc), by shortening a minor base of inverse trapezoid thereof one by one as said each cell wall is located more inwardly.
[5] A ceramic honeycomb structure according to the above [1] or [2], wherein any cell within a second end cell from a third cell to a fifth cell extending inwardly, taking a cell adjacent to the first end cell but located inward therefrom as a second starting cell, has such a cell wall thickness that a section of said each cell wall has such a spool shape as an inner side of spool is shorter than an outer side when the honeycomb structure is cut by a plane perpendicular to the direction of the cells (passages), and
a thickness of a cell wall having an shortest inner side is identical to the basic cell wall thickness (Tc), by shortening inner side of spool thereof one by one as said each cell wall is located more inwardly.
[6] A ceramic honeycomb structure according to the above [1], wherein there is a relation shown by a formula
1.10xe2x89xa6Tr1/Tcxe2x89xa63.00 
between the cell wall thickness (Tr1) of each outermost peripheral cell and the basic cell wall thickness (Tc), there is a relation shown by a formula
1.10xe2x89xa6(Tr1xcx9cTr3-20)/Tcxe2x89xa63.00 
between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr3-20) within a third end cell from a third cell to a twentieth cell extending inwardly, taking the outermost peripheral cell as a first starting cell,
a section of said each cell wall has such a rectangular shape as a minor side thereof is thickness of said each cell wall, or such an inverse trapezoidal shape as a minor base of inverse trapezoid is present inwardly and is thickness of said each cell wall, or such a spool shape as inner side of spool is shorter than outer side when the honeycomb structure is cut by a plane perpendicular to the direction of the cells (passages); and
a thickness of the cell wall having a shortest minor side, or a shortest minor base or a shortest inner side is identical to the basic cell wall thickness (Tc), by shortening the minor side of rectangle, or the inward minor base of inverse trapezoid or the inner side of spool one by one as said each cell wall is located more inwardly.
[7] A ceramic honeycomb structure according to any of the above [1] to [6], wherein there is the following relation
1.10xe2x89xa6(Tr1xcx9cTr3-20)/Tcxe2x89xa62.50 
between the basic cell wall thickness (Tc) and said each cell wall thickness (Tr1xcx9cTr3-20).
[8] A ceramic honeycomb structure according to any of the above [1] to [6], wherein there is the following relation
1.20xe2x89xa6(Tr1xcx9cTr3-20)/Tcxe2x89xa71.60 
between the basic cell wall thickness (Tc) and said each cell wall thickness (Tr1xcx9cTr3-20).
[9] A ceramic honeycomb structure according to any of the above [1] to [8], wherein the cells have a sectional shape of a triangle or a higher polygon.
[10] A ceramic honeycomb structure according to any of the above [1] to [9], wherein the honeycomb outer wall has a sectional shape of a circle, an ellipse, a trapezoid, a triangle, a tetragon, a hexagon or a special shape whose left and right are asymmetrical to each other.
[11] A ceramic honeycomb structure according to any of the above [1] to [10], wherein the honeycomb outer wall has a diameter of 144 mm or more when it has a circular sectional shape and, when it has a sectional shape other than a circular sectional shape, it has a sectional area equal to when it has a circular sectional shape, and,
there is the following relation
1.10xe2x89xa6(Tr1xcx9cTr10-40)/Tcxe2x89xa63.00 
between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr10-40) of cells existing within a first end cell from a third cell to a fortieth cell extending inwardly, taking the outermost peripheral cell as a first starting cell.
[12] A ceramic honeycomb structure according to any of the above [1] to [10], wherein the honeycomb outer wall has a diameter of 144 mm or more when it has a circular sectional shape and, when it has other than a circular sectional shape, it has a sectional area equal to when it has a circular sectional shape, and there is a following relation shown by a formula:
1.10xe2x89xa6(Tr1xcx9cTr10-30)/Tcxe2x89xa63.00 
between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr10-30) of cells within a first starting end cell from a tenth cell to a thirtieth cells extending inwardly, taking the outermost peripheral cell as a first starting cell.
[13] A ceramic honeycomb structure according to any of the above [1] to [12], wherein the cell walls and the honeycomb outer wall are made of at least one kind of materials selected from the group consisting of cordierite, alumina, mullite, silicon nitride, aluminum titanate (AT), zirconia and silicon carbide.
[14] A ceramic honeycomb structure according to any of the above [1] to [13], which is used as a carrier for catalyst for automobile exhaust gas purification.
[15] A ceramic honeycomb structure according to any of the above [1] to [14], which is assembled into a catalytic converter by loading a catalyst component on the cell walls and holding the honeycomb outer wall at the outer surface.
[16] A ceramic honeycomb structure according to any of the above [1] to [15], wherein the corners of each cell are formed so as to have a radius of curvature of 1.2 mm or less.
[17] A ceramic honeycomb structure according to any of the above [1] to [16], wherein each intersection between each outermost peripheral cell wall and the honeycomb outer wall is formed so as to have a radius of curvature of 1.2 mm or less.
[18] A ceramic honeycomb structure according to any of the above [1] to [17], wherein there is cell deformation and, when a diameter of the honeycomb structure is 120 mm or less, a first or third end cell is any of a third cell to a fifth cell and, when a diameter is more than 120 mm, a first or a third end cell is any of a sixth cell to a twentieth cell.
[19] A ceramic honeycomb structure according to any of the above [1] to [18], wherein there is provided with a corrugated cell wall having a corrugation in the direction of the cells (passages) between at least one pair of cells adjacent to each other, of the cells from the first starting cell to the first end cell or from the second starting cell to the second end cell or from the third starting cell to the third end cell.
As described above, the present invention can provide a ceramic honeycomb structure wherein the disadvantages incurred by the increased pressure loss and the decreased thermal shock resistance and the advantages brought about by the increased isostatic strength and the cell wall shape and honeycomb external shape of higher accuracy are balanced appropriately and which is suitably used, for example, as a carrier for catalyst for automobile exhaust gas purification.